This invention relates to trigger circuits for chemically augmented fuses of the type in which current flow through the fuse produces a magnetic field. More particularly, it relates to inductively coupling the trigger circuit to the magnetic field of the fuse to increase the inductance of the trigger circuit.
It is now known in the electrical fuse arts to use chemical augmentation to separate the fuse elements during current interruption. Typically, this augmentation takes the form of melting the fuse conductor in a number of locations along its length. The chemical augmentation may also be by way of mechanical separation rather than melting, with the means for mechanically cutting the fuse conductor propelled by a pyrotechnic chemical charge. For all of these types of fuses, the chemical augmentation action is initiated by a trigger circuit electrically connected in parallel with the fuse conductor. There are a number of such chemically augmented fuse designs known in the electrical fuse arts. For example, chemically augmented fuses are apparently described in U.S. Pat. No. 3,958,206, issued May 18, 1976 to R.V. Klint, U.S. Pat. No. 4,176,385 issued Nov. 27, 1979 to Dethlefsen, and U.S. Pat. No. 3,705,373 issued Dec. 5, 1972 to F.L. Cameron. Conventional chemically augmented fuses of the type which are useful in the present invention typically include at least one main fusible conductive element and an auxiliary conductive element electrically connected to the main element at at least two spaced-apart points along the length of the main element. The auxiliary element is made of a high resistivity material, so that current normally flows through the main fusible element. When, in response to an overcurrent, the main fusible element melts in the conventional manner at a location between the two points where the auxiliary element is connected, current is diverted into the auxiliary element. The auxiliary element comprises a trigger circuit and an exothermic material placed adjacent or touching the main fusible element. The trigger circuit and exothermic material may be combined, so that the auxiliary element is formed from exothermic material, or they may be separate, with the trigger circuit connected to the exothermic material so that current flow through the trigger circuit initiates a chemical reaction in the exothermic material. In either case, current flow through the trigger circuit, above a predetermined level, causes an exothermic reaction which heats the main fusible element and causes it to melt at one or more locations in addition to the first location which melted in response to the overcurrent.
It is often desirable that the trigger circuit used to initiate the chemically augmented operation of the fuse act only in response to a high current through the fuse, and not be triggered by a high rate of rise of current rather than a high level of current. In theory, if fuse and trigger conductors could be made having no inductance, the fuse voltage would essentially consist of only a resistive component, and the desired action of triggering only on a high current level could be achieved quite easily. In practice, however, the fuse conductor is typically helically wound around a cylindrical support structure, in order to increase the length of the fuse conductor for a given fuse size and thereby increase the fuse voltage capability. This winding pattern results in a substantial inductive component to the fuse impedance. Furthermore, in many fuses, multiple fuse conductors, all helically wound, are used to increase the current carrying capacity of the fuse.
The significance of this inductive component lies in the fact that the division of current between the main fuse conductor and the trigger conductor is crucial to the proper operation of the fuse. Because of the inductive voltage associated with this inductive component of the fuse conductor impedance, a high rate of rise of the level of the current through the fuse can result in a high voltage between the terminals of the fuse, even when the level of the current stays below the threshold current at which the fuse is designed to operate. With the trigger circuit electrically connected in parallel with the fuse conductor, this same fuse voltage is impressed on the trigger circuit. If the inductance of the trigger circuit is too low, the portion of the fuse voltage that is applied to the resistive component of the trigger circuit will be high enough to produce current flow through the trigger circuit in excess of the trigger level. Thus, unless the inductances of the fuse conductor and the trigger circuit are properly proportioned, the fuse may trip prematurely on a high rate of rise of current rather than just on high current.
Moreover, it is not practical to provide the proper proportioning by simply increasing the inductance of the trigger circuit. Because of the proportionalities involved, the trigger circuit inductance required to achieve the proper balance of inductance between the fuse conductor and the trigger circuit is quite large and not readily attainable for practical trigger conductors.
Accordingly, it is an object of the present invention to provide a trigger conductor for use in chemically augmented fuses that results in a constantly proportional division of current between the trigger circuit and the fuse conductor.
It is a further object of this invention to provide a trigger circuit for use in chemically augmented fuses that increases the inductance of the trigger circuit, without increasing the inductance of the trigger conductor.
It is also an object of the present invention to provide a chemically augmented fuse that is triggered only by a high current level, and not by a high rate of rise of current.